The present invention relates to an apparatus for displaying an image suspended in space.
Suspended images may be used, for example, as an aid to visualisation during laparoscopic (keyhole) surgery. The potential benefits of keyhole surgery are, on rare occasions, undermined by complications following certain keyhole procedures. A reason for this is the misinterpretation of the anatomy of the patient seen via a standard TV, or video monitor, as a twodimensional (2D) image. Another problem is that the monitor only allows the surgeon a xe2x80x9clook-upxe2x80x9d instead of a xe2x80x9cgaze-downxe2x80x9d position. This results in the surgeon no longer being able to see his hands while looking at the 2D image displayed on the monitor. It is therefore advantageous to suspend an image in space above the patient, enabling the surgeon to work in the preferred xe2x80x9cgaze-downxe2x80x9d position.
Apparatus for displaying an image suspended in space, which may be used for keyhole surgery, is disclosed in PCT/GB98/00186 and European Patent No. EP 0460 873 (Thorn EMI plc). The former apparatus comprises an object for display, a retro-reflector, a beam-splitter, and a converging optical element. The converging optical element may comprise a Fresnel lens.
A Fresnel lens generally has a micro-structured surface on one side, and a flat or smooth surface on the other, as shown in FIGS. 1 and 2. The micro-structured surface consists of a series of grooves (22) with changing slope angles as the distance from the optical axis increases. The grooves are arranged in concentric circles, the circles having a point of central symmetry (26).
The use of a Fresnel lens in a suspended image system gives rise to unwanted images formed by reflections from the surfaces of the lens. Two types of reflections are formed: those from the grooved surface (xe2x80x98floating reflectionsxe2x80x99), and those from the flat or smooth surface (xe2x80x98fixed reflectionsxe2x80x99). Reflections from the grooved surface of the Fresnel lens (which is normally situated facing the object to be displayed) give rise to a virtual image of the object. Reflections from the flat or smooth surface of the lens give rise to a real image (28) of the object. These reflected images are formed in addition to the image of the object formed by the refraction of light from the Fresnel lens.
Suspended image systems may be used for applications in which the presence of these reflected images is very distracting, as, for example, in keyhole surgery. Application of anti-reflective coatings to the Fresnel lens can significantly reduce the visibility of these unwanted images. However, such coatings can be very expensive and difficult to apply, particularly to the grooved surfaces of the lens. The coatings may also cause colour shifts in the suspended image.
An aim of the present invention is to improve the viewability of a suspended image by shifting the position of the reflected images such that they are no longer visible when viewing the suspended image.
According to a first aspect of the invention there is provided apparatus for displaying a suspended image comprising: an object for display; a first retro-reflector being arranged to receive light from the object; a beam-splitter in a path for light from the object to the first retro-reflector, the beam-splitter being arranged at an oblique angle to the direction of propagation of the light such that it transmits part of the light and reflects part of the light, the light being transmitted or reflected by the beam-splitter is received by the first retro-reflector and is reflected by it back to the beam-splitter, which either reflects or transmits part of this retro-reflected light, to form a real image suspended in space; a converging optical element arranged between the beam-splitter and the first retro-reflector such that in use light from the beam-splitter passes through the converging optical element to the first retro-reflector, which reflects it back through the optical element towards the beam-splitter, characterised in that the converging optical element comprises a Fresnel lens having a substantially flat or smooth surface and a grooved surface, the arrangement of grooves of the grooved surface having no point of central symmetry.
A further retro-reflector may be provided to receive light being reflected by the beam-splitter from the object, such that part of the light being reflected from both retro-reflectors forms the real image suspended in space.
A further Fresnel lens may be provided in the path of light from the beam-splitter to the further retro-reflector. Preferably the Fresnel lens or lenses is/are located such that they form an image of the given object or image at the retro-reflector(s).
The Fresnel lens(es) may comprise an array of smaller Fresnel lenses, each smaller Fresnel lens having no point of central symmetry. The array of smaller Fresnel lenses collects the light which would have been collected by the single Fresnel lens. The individual lenses of the array may all face the beam-splitter so that they share a common focal plane. The focal length of such xe2x80x9ctiledxe2x80x9d lenses can be made very shortxe2x80x94of the order of one or two centimetersxe2x80x94which enables the lens and the retro-reflector to be placed close together. This allows the apparatus to be more compact. Such lens arrays combine good quality optical performance, a large area over which to gather light, and a short focal length compared with the lateral dimension of the light gathering area.
Preferably at least one surface of the Fresnel lens (or lenses) is coated with an anti-reflective coating in order to render the fixed reflection of the object substantially invisible to a viewer.
The object to be displayed may be an image being displayed by a visual display unit. Alternatively, an illuminated solid object may be used in place of the visual display unit.
The retro-reflector(s) may be composed of an array of very small, or micro, retro-reflectors. The individual retro-reflectors may be of the order of 100 xcexcm. The retro-reflector(s) may comprise a sheet of bead type retro-reflective material. Alternatively, the retro-reflective material may comprise a sheet of a comer cube retro-reflector material, a triple mirror retro-reflector, or other direction selective screen.
According to a further aspect of the invention, the retro-reflector(s) may be replaced by a specularly reflective surface which forms a real or virtual image suspended in space depending upon the distance between the object and the converging optical element. Alternatively, a layer such as a mirror (e.g. plane, parabolic or spherical) may be used in place of the retro-reflector(s). The specularly reflective surface or layer may be partly light transmissive, or opaque, and may be curved or flat.
Preferably the beam-splitter makes an angle of between 35 and 55 degrees to the direction of propagation of light from the object to the first retro-reflector or specularly reflective surface.
Suspended images having the best resolution are formed when the optical distances between the object and the converging lens, and the converging lens and the retro-reflector, are selected so that the converging lens produces an image of the object on or at the surface of the retro-reflector. The apparatus is therefore preferably arranged such that the distance between the retro-reflector(s) or specularly reflective layer and the converging optical element(s) is selectable.